1. Field of the Invention
This invention relates to a focal device for a telescope, particularly one able to keep the axial lines of the ocular and the telescope aligned to obtain an absolutely precise focus by adjusting the focus.
2. Description of the Prior Art
A focal device of the conventional telescope, as shown in FIGS. 1-3, has a structure that, having not been improved any until now, has an eyepiece connecting tube 11, and an eyepiece 12 fitted at the front end of the combining tube 11 of an eyepiece tube 10. The conjunction of the eyepiece tube 10 and the eyepiece connecting tube 11 counts on a rack 110, set on the bottom of the eyepiece connecting tube 11 and meshing with a gear 100 set on the bottom of the eyepiece tube 10. Then the gear 10 is activated by a rotary wheel 101 to rotate at its site, as shown in FIG. 2. So, when the focus is to be carried out, we need only to rotate the rotary wheel 101 that can activate the gear 100 to drag the rack 110 to move to and fro, making the ocular connecting tube 11 and the ocular 12 move together synchronously. The conventional focal device, though be able to keep the ocular connecting tube 11 and the ocular 12 moving to and fro, has a practical defect, unable to adjust the focus precisely. The reason is described as below:
The precision of focusing is dependent on if the telescope axial line (A) overlaps totally with the ocular axial line (B) or not. As the meshing position between the eyepiece connecting tube 11 and the gear 100 locates below the telescope axial line (A) and the eyepiece line (B) the supporting pivotal point of the eyepiece connecting tube 11 and the gear 100 are below the two axial lines (A), and (B), the eyepiece axial line (B) always remains bias, not overlapping totally with the telescope axial line (A) despite rotating the gear wheel 100 to move forwards, as shown in FIG. 2, or to move backwards, as shown in FIG. 3. Therefore, a precise focus can never be obtained.